Method for programming a multilevel phase change memory device

ABSTRACT

A method of programming a phase change device includes selecting a desired threshold voltage (Vth) and applying a programming pulse to a phase change material in the phase change device. The applying of the programming pulse includes applying a quantity of energy to the phase change material to drive at least a portion of this material above a melting energy level. A portion of the energy applied to the phase change material is allowed to dissipate below the melting energy level. The shape of the energy dissipation from the phase change material is controlled until the energy applied to the phase change material is less than a quenched energy level, to cause the phase change device to have the desired Vth. A remaining portion of the energy applied to the phase change material is allowed to dissipate to an environmental level.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/976,648, filed on Oct. 29, 2004. The disclosure of this priorapplication from which priority is claimed is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to memory systems, and moreparticularly, to methods, systems and apparatus for programming phasechange memory cells.

2. Description of the Related Art

The general concept of utilizing electrically writable and erasablephase change materials (i.e., materials that can be electricallyswitched between generally amorphous and generally crystalline states)for electronic memory applications is well known in the art. A typicalphase change material is a material that has two general states: agenerally amorphous state and a generally crystalline state. The phasechange material can include one or more chalcogenide compounds that atleast partly include one or more of the following materials: Te, Se, Sb,Ni, and Ge and various combinations thereof.

The typical phase change material can be switched from one state to theother by passing an electrical current or other type of energy throughthe phase change material to cause it to change states. Typically in thefirst state (e.g., amorphous state), the phase change material has arelatively high resistance and in the second state (e.g., thecrystalline state), the phase change material has a relatively lowresistance. The resistance ratio between amorphous state and crystallinestate is about 1000:1.

As the state of the phase change material can only be changed by asufficient application of energy (e.g., a programming energy pulse),then the phase change material is generally non-volatile in that it doesnot require energy to maintain it's current state. Further, because theresistance of the phase change material varies with the state (e.g., alow resistance at for a crystalline state and a high resistance for anamorphous state), then the phase change material can be reliably used tostore binary data such as may be used for a memory cell in a computer orother binary data storage usage.

The programming energy pulse determines the actual resistance of aprogrammed phase change device. By way of example, a first programmingenergy pulse (i.e., 1 ms pulse of 1.42 mA) is applied to a phase changedevice and results in a resistance of 50 ohms. If a second programmingenergy pulse (i.e., 1 ms pulse of 1.98 mA) applied to the same phasechange device a resistance of 500 ohms could result. As a result onlyvery slight variations in the amount of energy (e.g., electrical currentin this instance) results in markedly different resistance levels of thephase change device. Further, as the process variations (e.g., filmthickness of the phase change material) and the operating parameters(e.g., operating temperature, voltage, etc.) vary, then the energyrequired to achieve the desired resistance level changes. Therefore, itis difficult to accurately program a phase change device to a selectedresistance level.

In view of the foregoing, there is a need for a system and method foraccurately and quickly programming multiple data values in a phasechange device.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing asystem and method for accurately and quickly programming multiplethreshold voltage levels in a phase change device. It should beappreciated that the present invention can be implemented in numerousways, including as a process, an apparatus, a system, computer readablemedia, or a device. Several inventive embodiments of the presentinvention are described below.

One embodiment provides a method of programming a phase change deviceincludes selecting a desired threshold voltage (Vth) and applying aprogramming pulse to a phase change material in the phase change device.Applying the programming pulse includes applying a quantity of energy tothe phase change material to drive at least a portion of the phasechange material above a melting energy level. The quantity of energy isapplied for a first time interval. A portion of the energy applied tothe phase change material is allowed to dissipate below the meltingenergy level. A shape of energy dissipation from the phase changematerial is controlled until the energy applied to the phase changematerial is less than a quenched energy level. The shape of the energydissipation is controlled to cause the phase change device to have thedesired Vth. A remaining portion of the energy applied to the phasechange material is allowed to dissipate to an environmental level.

The energy applied to the phase change material includes at least one ofelectrical current, heat, light, voltage. The desired Vth levelcorresponds to a desired data value. The desired Vth level is one ofmultiple desired Vth levels and each one of the desired Vth levelscorresponds to one of a multiple desired data values.

The quantity of the energy applied to the phase change material can be afunction of an operating parameter. The operating parameter includes atleast one of an operating temperature and an input voltage. The functionof the operating parameter can compensate for a variation in theoperating parameter.

The quantity of the energy applied to the phase change material can alsobe a function of a phase change manufacturing process variable. Thephase change manufacturing process variable includes at least one of agroup consisting of a phase change material type, a film thickness, anda phase change contact size. The function of the phase changemanufacturing process variable can compensate for a variation in thephase change manufacturing process variable.

Controlling the shape of the energy dissipation from the phase changematerial until the energy applied to the phase change material is lessthan the quenched energy level can also include applying multiple energysub-pulses. The sub-pulse can have a profile of at least one of asquare, a triangle, one or more stairs, a trapezoid, a trapezium, asquare with straight sloping pattern, a square with u-shaped tailpattern, a square with a reverse u-shaped tail pattern, and acombination thereof.

The programming pulse can have a profile of at least one of a square, atriangle, one or more stairs, a trapezoid, a trapezium, a square withstraight sloping pattern, a square with u-shaped tail pattern, a squarewith a reverse u-shaped tail pattern, and a combination thereof. Thephase change material can include a chalcogenide material.

Another embodiment provides a method of programming a multi-state phasechange device that includes selecting a desired threshold voltage (Vth)from a multiple desired Vths. Each one of the desired Vths correspondingto one of a multiple data values. A programming pulse is also applied toa phase change material in the phase change device including applying aquantity of energy to the phase change material to drive at least aportion of the phase change material above a melting energy level. Thequantity of energy being applied for a first time interval. A portion ofthe energy applied to the phase change material is allowed to dissipatebelow the melting energy level. A shape of energy dissipation from thephase change material is controlled until the energy applied to thephase change material is less than a quenched energy level. The shape ofthe energy dissipation is controlled to cause the phase change device tohave the desired Vth. A remaining portion of the energy applied to thephase change material is allowed to dissipate to an environmental level.

The quantity of the energy applied to the phase change material is afunction of an operating parameter. The quantity of the energy appliedto the phase change material is a function of a phase changemanufacturing process variable.

Yet another embodiment provides a method of programming a memory array.The method includes selecting a desired threshold voltage (Vth) andapplying a programming pulse to a phase change memory device in thememory array. A steering element can apply the programming pulse to thephase change memory device including applying a quantity of energy tothe phase change device to drive at least a portion of a phase changematerial in the phase change device above a melting energy level. Thequantity of energy is applied for a first time interval. A portion ofthe energy applied to the phase change material is allowed to dissipatebelow the melting energy level. A shape of energy dissipation from thephase change material is controlled until the energy applied to thephase change material is less than a quenched energy level. The shape ofthe energy dissipation is controlled to cause the phase change device tohave the desired Vth. A remaining portion of the energy applied to thephase change material is allowed to dissipate to an environmental level.

The desired Vth level is one of multiple desired Vth levels and each oneof the desired Vth levels corresponds to one of multiple desired datavalues. The quantity of the energy applied to the phase change materialcan be a function of an operating parameter. The quantity of the energyapplied to the phase change material can be a function of a phase changemanufacturing process variable.

The disclosed invention provides the advantage of being able toaccurately compensate for process variations, operating parametervariations, and accurately programming different resistance levels in aphase change device.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings.

FIG. 1 shows a phase change memory cell array.

FIGS. 2A-2H show various programming energy pulse profiles, inaccordance with one or more embodiments of the present invention.

FIG. 3 shows a programming pulse profile, in accordance with oneembodiment of the present invention.

FIG. 4 shows two views of the same programming pulse, in accordance withone embodiment of the present invention.

FIG. 5 shows a combination pulse profile, in accordance with oneembodiment of the present invention.

FIG. 6 is a flowchart of the method operations of programming a phasechange device in accordance with one embodiment of the presentinvention.

FIG. 7 shows a graphical representation of multiple Vth levels that canbe programmed into a phase change device, in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Several exemplary embodiments for a system and method for accurately andquickly programming multiple threshold voltage levels in a phase changedevice will now be described. It will be apparent to those skilled inthe art that the present invention may be practiced without some or allof the specific details set forth herein.

Phase change devices can be read and programmed very quickly and do notrequire power to maintain their state. Therefore, phase change devicesare very useful devices for storing data (e.g., as a computer memorydevice). Further, because a wide range of different threshold voltagesexist between the amorphous state and the crystalline state, then thethreshold voltage can be separated into multiple levels and each one ofthe multiple threshold voltage levels could be used to indicate adifferent data value that is stored in the phase change device.

One embodiment provides a system and method for a precise selection ofeach one of the multiple threshold voltage levels for a phase changedevice. The precise selection of the multiple threshold voltage levelsfor the phase change device therefore provides a precise control of theselection of the threshold voltage (Vth) level required that correspondsto each one of the multiple data values that can be stored in the phasechange device. The precise selection of each one of the multiplethreshold voltage levels for the phase change device can also provide animproved programming method that reduces the variation of Vth. Thereduced variation of Vth increases stability in different operatingconditions (e.g., temperature, current, input voltage, etc.) and canalso compensate for semiconductor manufacturing process variations(e.g., film thickness variations, phase change material variations,phase change contact size, etc.).

In one embodiment, the threshold voltage of a phase change device can beselected for a relatively small number (e.g., 4) of multiple resistancelevels. For example, the resistance levels can include four levels ofresistance (e.g., 5 k, 50 k 500 k and 5M ohm). However, it can bedifficult to accurately detect each of the different resistance levels.By way of example, if 0.1 V read voltage is applied on a phase changedevice with resistance at the four different states described above, theresulting current would be 20 uA, 2 uA, 0.2 uA, and 20 nA, respectively.It can be very difficult to accurately detect all four of the differentVth levels as 0.2 uA is a very low current, and 20 nA is the same orderof magnitude of current flow as noise in a typical phase change devicecircuit.

The resistance level, and therefore the Vth level, of a phase changedevice can be decided by the state of the phase change material in thephase change device. The state of phase change material is a descriptionof a quantity of the phase change material that is changed from theamorphous state to the crystalline state. The quantity of the phasechange material that is changed determines the resistance of phasechange material and the threshold voltage of the phase change device.The state of the phase change material is mostly determined by thequench rate (i.e., how rapidly the phase change material cools from amelted state back to a solid form. However, the quench rate is alsostrongly dependant on the operating conditions (e.g., temperature, inputvoltage, etc.) and process variations (e.g., film thickness, phasechange contact size, etc.). In one embodiment, the profile of theprogramming energy pulse can quench the phase change material in thephase change device to a produce a desired threshold voltage level, withminimum sensitivity to the operating conditions and process variations.

Below threshold voltage (Vth) the current flow through the phase changedevice is very low. Conversely, above Vth, the current flow through thephase change device is much larger. By way of example, the typicalon/off ratio can be greater than about 1000 times. The currentdifference between on and off conditions is very clear. By sensing thecurrent difference, the on or off condition of the phase change devicecan be easily and quickly distinguished.

One embodiment provides a programming method that can achieve preciseVth control. FIG. 1 shows a phase change memory cell array 100. Fourmemory cells 110A-110D are included in the memory cell array 100. Thememory cell 110A includes a transistor 102, as a steering element, andone phase change device 104 as a memory element. Turning on the steeringelement 104 and applying sufficient programming energy pulse can programthe phase change device 102.

The programming energy pulse that has a profile that describes the shapeof the pulse. FIGS. 2A-2H show various programming energy pulse profiles205-240, in accordance with one or more embodiments of the presentinvention. The programming energy pulse profiles 205-240 have thevoltage shown in the vertical axis and time on the horizontal axis. Itshould be understood that the vertical axis could also show the current,energy, heat, light or other type of energy of the respective pulseprofiles 205-240. The profile can be square 205, triangle 210, stairs215, trapezoid 220, trapezium 225, square with straight sloping trailingedge 230 or tail pattern, square with u-shaped tail pattern 235, or asquare with a reverse unshaped tail pattern 240, or other complexpatterns.

The voltage of the programming energy pulse can be from about 0.01 V toabout 20 V. The waveform (i.e., shape or profile) of the programmingenergy pulse can be from about 1 ns to about 10 us and from about amaximum energy amplitude to a minimum energy amplitude. Each differentprogramming energy pulse can result in a different resistance level anda different Vth.

By way of example, a programming energy pulse with a square profile witha straight sloping tail pattern (e.g., profile 230 of FIG. 2F). Theduration of the main square portion of the pulse (i.e., from t0 to t1)can range from about 0 ns to about 1000 ns. By changing the slope (i.e.,shape) of the tail portion (i.e., from t1 to t2 as shown in programmingenergy pulse 230), the resistance level and the Vth of the phase changedevice 102 can be accurately selected. By way of example, theprogramming energy pulse 205 has vertical tail and results in a firstresistance level and a first Vth. In contrast, the programming energypulse 230 has the same voltage as pulse 205 but also has a straight,sloping tail that extends from t1 to t2 as opposed to the vertical tailon pulse 205. Programming energy pulse 230 results in a secondresistance level and a second Vth.

FIG. 3 shows a programming pulse profile 300, in accordance with oneembodiment of the present invention. The programming pulse profile 300has three portions: melting 305, phase change 310, and the solid cooling315. In the melting portion 305, between t1 and t2, the input pulseprovides sufficient energy to cause the phase change material to melt.In the phase change portion 310, between t1 and t2, the applied energyis reduced (as compared to the melting portion 305) and the phase changefilm is quenched at a selected cooling speed. The cooling speed iscontrolled by the shape of the pulse as phase change material cools fromthe melting temperature 320 to the quenched temperature 325, and thusdetermines the resistance level and the Vth of the phase change device.In the solid cooling portion 315, between t2 and t3, the phase changematerial cools to the operating temperature of the operatingenvironment. Since the operating temperature is lower than the meltingtemperature 320 of the phase change device and also cooler than thequenched temperature 325, then the resistance level of the phase changedevice is not significantly changed from the previous phase changeportion 310.

The resistance level and Vth is determined by the shape of the energypulse during the cooling portion 315 and therefore the environmentaltemperature or the given energy can also impact how the phase changedevice is quenched. In one embodiment, the programming pulse profile isselected so that the cooling portion 315 is controlled independent fromthe operation conditions and process variations. By precisely selectingthe cooling portion 315, different resistance levels and different Vthscan be selected.

A square programming pulse 205 of FIG. 2A above is difficult to use forselecting different resistance levels and Vths because the quenching ofthe phase change device is highly dependant on the environmentalconditions. FIG. 4 shows two views 405, 410 of the same programmingpulse, in accordance with one embodiment of the present invention. Inboth views 405 and 410, the pulse height and durations are the same.However, in view 410, the operating temperature is elevated as comparedto view 405. As a result, the amount of energy 420′ required to reachmelting temperature is less in view 410 as compared to energy level 420in view 405. Further, more energy must be dissipated to fall below theenergy level 420′. As a result, the interval between t0 and t1′ of view410 is longer than the interval between t0 and t1 of view 405. Further,the amount of energy required to be dissipated from peak energy toquenched energy level 425′ is also increased in view 410 as compared toview 405. As a result, as the environmental conditions change, adifferent resistance level and a different Vth can result from the sameprogramming pulse profile 405 and 410. The quench energy shape can becontrolled by the programming pulse profile to compensate for variationsin environmental conditions such as operating temperature.

By way of example, the slope of the tail portion (i.e., between time t1and time t2 and between melting energy level 420 and quenched energylevel 425 of the programming pulse profile) can be tightly controlled toselect a desired resistance level and Vth that is independent of theoperating conditions. In a stair-stepped profile 215 or a sloping tailsuch as in pulse profiles 210, 225, 230, 235, 240, time or duration ofthe tail portion of the programming pulse profile is a significantcomponent. Conversely, the tail portion of the programming pulse 205 isalmost non-existent as tailing edge of the pulse is mostly vertical.Therefore, the sloping tail of pulse profiles 210, 225, 230, 235, 240provide a control element to the select the resulting resistance leveland Vth of the phase change device.

Aspects (e.g., duration, stair height, number of steps, step duration,combinations of different tail profiles, etc.) of the sloping tail ofpulse profiles 210, 225, 230, 235, 240 can be manipulated to select theresulting resistance level and Vth of the phase change device. It shouldbe noted that the resistance level and Vth of the phase change devicecan be selected independently of one another. Restated, a first energypulse profile can result in a first resistance level and a first Vthlevel of the phase change device. A second energy pulse profile canresult in the first resistance level and a second Vth level of the phasechange device. Similarly, a third energy pulse profile can result in asecond resistance level and the first Vth level of the phase changedevice.

FIG. 5 shows a combination pulse profile 500, in accordance with oneembodiment of the present invention. The combination pulse profile 500includes a high temperature melting portion 505 that is bounded by themelting energy level 520 and between t0 and t1. Multiple sub-pulses530-536 of energy are applied to maintain or extend the time that thephase change material is within the phase change portion 510. Themultiple sub-pulses 530-536 of energy allow a desired resistance andcorresponding Vth to be selected. The cooling portion 515 allows thephase change material to cool to the environmental operatingtemperature.

FIG. 6 is a flowchart of the method operations 600 of programming aphase change device in accordance with one embodiment of the presentinvention. In an operation 605, a desired resistance level and/or adesired Vth is selected. In an operation 610, a programming pulse isapplied to the phase change material. The programming pulse includes amelting portion, a phase change (i.e., quenching) portion and a coolingportion. In operation 610, the melting portion of the programming pulseis applied to the phase change material. In an operation 615, a portionof the energy applied to the phase change material is allowed todissipate below a melting energy level.

In an operation 620, a shape of the energy dissipation from the phasechange material is controlled so that the selected resistance leveland/or a selected Vth is achieved. In an operation 625, the energy isallowed to dissipate below a quenched energy level. In an operation 630,the energy is allowed to dissipate to the level of the environment(i.e., operating temperature) and the method operations can end.

FIG. 7 shows a graphical representation 700 of multiple Vth levels thatcan be programmed into a phase change device, in accordance with oneembodiment of the present invention. The normalized current is shown onthe vertical axis and the normalized Vth is shown on the horizontalaxis. As shown by the graphs 702-712, when the respective Vth level714-720 is exceeded the current flow through the phase change deviceincreases approximately vertically. By way of example, graph 702 showsthat when a first Vth level 714 is met, the current flow through thephase change device increases approximately vertically. Similarly, if asecond Vth level 716 is desired, then the phase change device can beprogrammed so that current flow increases at the second Vth level 716.As described above, a Vth can be selected and therefore the Vth rangebetween a low Vth level 714 and a high Vth level 720 can be divided intomultiple Vth levels, each one of the multiple Vth levels can indicate adifferent data value. As described above, a phase change device can beused as a multi-state device.

It should be understood that while the phase change device 102 is shownin an array structure in FIG. 1 above, it should be understood that thepresent invention is not limited to phase change devices in an arraystructure or even phase change devices paired with a steering element(e.g., steering element 104). The present invention can be used toselect and control a resistance level and a Vth level of any type ofphase change device. The impact of operation condition and processvariation can also be minimized and/or compensated for.

As used herein in connection with the description of the invention, theterm “about” means +/−10%. By way of example, the phrase “about 250”indicates a range of between 225 and 275. With the above embodiments inmind, it should be understood that the invention may employ variouscomputer-implemented operations involving data stored in computersystems. These operations are those requiring physical manipulation ofphysical quantities. Usually, though not necessarily, these quantitiestake the form of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. Further, themanipulations performed are often referred to in terms, such asproducing, identifying, determining, or comparing.

Any of the operations described herein that form part of the inventionare useful machine operations. The invention also relates to a device oran apparatus for performing these operations. The apparatus may bespecially constructed for the required purposes, or it may be ageneral-purpose computer selectively activated or configured by acomputer program stored in the computer. In particular, variousgeneral-purpose machines may be used with computer programs written inaccordance with the teachings herein, or it may be more convenient toconstruct a more specialized apparatus to perform the requiredoperations.

The invention can also be embodied as computer readable code on acomputer readable medium. The computer readable medium is any datastorage device that can store data that can thereafter be read by acomputer system. Examples of the computer readable medium include harddrives, network attached storage (NAS), read-only memory, random-accessmemory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer systems so that thecomputer readable code is stored and executed in a distributed fashion.

It will be further appreciated that the instructions represented by theoperations in the above figures are not required to be performed in theorder illustrated, and that all the processing represented by theoperations may not be necessary to practice the invention. Further, theprocesses described in any of the above figures can also be implementedin software stored in any one of or combinations of the RAM, the ROM, orthe hard disk drive.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1. A system comprising a memory array including a plurality of memorycells, each memory cell comprising a memory element made of a phasechange material, which has a selectable electrical property controlledby an energy imparting and dissipating process between a melting energylevel and a quench energy level.
 2. The system of claim 1, wherein theselectable electrical property is a threshold voltage.
 3. The system ofclaim 1, wherein the selectable electrical property is a resistancelevel.
 4. The system of claim 2, wherein the threshold voltagecorresponds to a data value.
 5. The system of claim 1, wherein theenergy imparting and dissipating process is a function of an inputenergy.
 6. The system of claim 5, wherein the input energy has a profileof at least one of a square, a triangle, one or more stairs, atrapezoid, a trapezium, a square with a straight-sloped tail pattern, asquare with a unshaped tail pattern, a square with a reverse u-shapedtail pattern, and a combination thereof.
 7. The system of claim 5,wherein the input energy includes at least one of electrical current,heat, light, and voltage.
 8. The system of claim 1, wherein the energyimparting and dissipating process is a function of an operatingtemperature.
 9. The system of claim 1, wherein the phase change materialincludes a chalcogenide material.
 10. The system of claim 1, wherein theenergy imparting and dissipating process further includes beingcontrolled to exceed the melting energy.
 11. The system of claim 1,wherein the energy imparting and dissipating process further includesbeing controlled to diminish to an environmental level.
 12. A memorycell comprising a steering element and a memory element made of a phasechange material, which has a selectable electrical property determinedby a profile of energy application during a cooling process from amelting energy level to an environmental level.
 13. The memory cell ofclaim 12, wherein the steering element is a transistor.
 14. The memorycell of claim 12, wherein the selectable electrical property is athreshold voltage.
 15. The memory cell of claim 14, wherein thethreshold voltage corresponds to a data value.
 16. The memory cell ofclaim 12, wherein the selectable electrical property is a resistancelevel.
 17. The memory cell of claim 12, wherein the energy applicationincludes at least one of electrical current, heat, light, and voltage.18. The memory cell of claim 12, wherein the phase change materialincludes a chalcogenide material.
 19. The memory cell of claim 12,wherein the profile of energy application includes at least one of asquare, a triangle, one or more stairs, a trapezoid, a trapezium, asquare with a straight-sloped tail pattern, a square with a u-shapedtail pattern, a square with a reverse u-shaped tail pattern, and acombination thereof.
 20. The memory cell of claim 12, wherein theprofile of energy application during the cooling process includes atleast one of a straight-sloped contour, a u-shaped contour, a reverseu-shaped contour, a stepwise decreasing contour, a contour of stepfunction, and a combination thereof.